Quickbar 3 is in the middle between your and your targets health bars and entails your offensive and defensive cooldowns, so that you have a better eye on them.
Note that Num1 and Num 3 (for Force Potency and the Adrenal) in my Screenshot are actual bound to my mouse. The game doesn't register two of its Keys so I bound them to the num block.

Quickbar 4 are minor utiliy skills. Slot 1 and 5 are also on my mouse and are Project and Vindicate (fillers I rarely need). The rest is just stuff I don't need to bind to a key due to it's rare necessity in fights.